I encountered the same problem as described above (i.e. losing the IP address on my wife's G3 iBook running 10.3.9 after updating iTunes and Safari). I followed both sets of procedures listed above and together they resolved my issue.
More precisely, following "dplfx":
http://discussions.apple.com/thread.jspa?threadID=309223&tstart=0
1) Move iTunes onto the Desktop.
2) Run Disk Utility
3) Go to HD>Library>Receipts and trash all iTunes .pkg files EXCEPT for iTunesX.pkg and iTunesPhoneDriver.pkg
4) empty Trash.
5) move iTune from Desktop back to 'Applications'.
6) restart after you finish the steps above & do the repair & verify again
Then following Eric S.:
(1) restart computer
(2) don't click on the airport icon but rather go to 'system pref', 'network', and the click on 'airport' and 'configure'.
(3) change from 'automatic' to 'join a specific network'
(4) at this point, it connected to my router, but still wouldn't assign an IP and get me online. so, I....
(5) restarted again...everything ok.
This seemed to resolve the issue. Not sure what I did but nothing else worked.